Weapons such as rifles or pistols may include an optical (or telescopic) sighting device having optical components that magnify an image of a target. The optical sighting device typically includes a group of lenses disposed in a housing which is attached to the rifle. The lenses may be fixed in position relative to each other to provide fixed magnification, or may be movable to achieve variable or a range of magnification. Variable magnification of an image may allow the telescoping sighting device to be used in a wider range of applications. For example, a lower magnification may allow a user to scan the terrain for a particular object, while a higher magnification may allow the user to view the object in more detail. For uses in military, police and hunting applications, compact and lightweight optical sighting devices having a high magnification ratio (or zoom ratio) are desired. Magnification ratio or zoom ratio refers to the ratio of the highest and the lowest magnification provided by the optical sighting device.
FIG. 1 illustrates one example of an optical sighting device 10 having a variable magnification ratio, as described in U.S. Pat. No. 7,944,611 titled “HIGH ZOOM RATIO OPTICAL SIGHTING DEVICE” and issued on May 17, 2011. The optical sighting device 10 includes an erector system including a first lens erector 40 and a second lens erector 60, an objective lens 15, a prism erector assembly 25, an eyepiece 80 and three focal plains 30, 55 and 75. The lenses included in both the first and second lens erectors, 40 and 60 are moveable along an optical axis 12 to provide variable magnification. Since the erector assemblies each invert the image, the prism erector assembly 25 is needed to correct the inverted image and to produce an image in the same orientation as the object being imaged. The optical sighting device 10 may provide a magnification ration of approximately 10:1 with the total length of the optical sighting device 10 being approximately 14 inches. To achieve a higher magnification ratio (for example, approximately 27:1), the prism erector assembly 25 may be removed, and a third movable lens erector assembly may be added behind the focal plane 75. However, the addition of the third lens erector adds a fourth focal plane to the system and results in increased length of the overall optical sighting device by approximately 1-4 inches. Furthermore, the additional lens components increase the overall complexity and weight of the optical sighting device.